1. Field of the Invention
The present invention relates to a preparation method for acrylonitrile-butadiene-styrene (ABS) resin, more precisely, a preparation method for rubber-modified styrene copolymer resin having high impact strength and gloss, in which rubber is mixed with a monomer at the ratio of 5–10 weight % to 100 weight % of monomer in a graft reactor at the early reaction stage to regulate the graft reaction between rubber and monomer.
2. Discussion of Related Art
Preparation methods of ABS resin, a rubber-modified styrene copolymer resin, are exemplified by emulsion polymerization, suspension polymerization, solution polymerization, bulk polymerization, a joint use of suspension polymerization and bulk polymerization, and a joint use of emulsion polymerization and bulk polymerization. Among these methods, emulsion polymerization and bulk polymerization are most widely used, and in particular emulsion polymerization is most preferred. ABS resin prepared by the emulsion polymerization has good mechanical properties and gloss resulted from the average size of 0.2–1.5 μm of rubber particles that exist in dispersed phase in continuous phased styrene-acrylonitrile copolymer (SAN copolymer). However, those physical properties might be decreased with the use of emulsifier and coagulant, which are necessary for the emulsion polymerization process. That is, emulsifier and coagulant cannot be completely eliminated during the coagulation and dehydration processes, and remain in the final product, resulting in the decrease of physical properties of the product. In addition, contaminated water used as a polymerization medium can not be simply discarded. And, compared with bulk polymerization, a kind of a continuous process, the emulsion polymerization needs coagulation and dehydration processes separately after polymerization, so the emulsion polymerization is non-economic.
The method for production of ABS resin by continuous bulk polymerization is described in U.S. Pat. Nos. 3,337,650, 3,511,895, and No. 4,417,030. Bulk polymerzation is a kind of polymerization in which styrene monomers and acronitrile monomers are dissolved in a reaction medium according to a certain ratio, butadiene rubber or styrene-butadiene rubber is dissolved in the above mixture, and a proper amount of reaction initiator, molecular regulator and other additives are added to the reaction solution above, followed by heating. As the bulk polymerization proceeds, SAN copolymer is produced by styrene monomers and acrylonitrile monomers, and these styrene monomers and acrylonitrile monomers are reacted with dissolved butadiene rubber or styrene-butadiene rubber to produce graft SAN copolymer. The prepared SAN copolymer is not mixed with rubber dissolved in the reaction medium from the beginning of the reaction and makes two phases, making the total polymerization solution heterogeneous phase. Even under such heterogeneous phase, rubber dissolved in the polymerization solution is in continuous phase in the early reaction stage showing low conversion rate, but once the phase volume of copolymer of styrene or styrene derivatives and acrylonitrile or acrylonitrile derivatives, which is increased as reaction continuous, is bigger than that of rubber in the polymerization solution, the copolymer makes continuous phase. This phenomenon is called ‘phase inversion’ and the time point when the volume of the copolymer is equal to the volume of rubber is called phase inversion time. After phase inversion, rubber phase becomes dispersed phase, suggesting that rubber particles are included in the final resin product.
This ABS resin product has excellent plasticity, dimensional stability and impact resistance, so that it has been applied to the various industrial fields such as electric appliances, office machine parts, motorcar parts, etc. The mechanical properties and gloss of ABS resin are affected by the size, composition and content of rubber particles included in the resin, for example when the content of rubber particles in resin is low or the sizes of the rubber particles are small, tensile strength and gloss are improved but elongation and impact strength are reduce. On the contrary, when the content of rubber particles in resin is increased, the impact strength is enhanced but gloss is degraded. Such problems are attributed to the difference in graft reaction of butadiene rubber, styrene monomer and acrylonitrile in the early reaction stage. Styrene-acrylonitrile resin, a matrix of ABS resin, is incompatible against butadiene rubber forming dispersed phase. Monomer grafting butadiene rubber lies in between butadiene rubber and styrene-acrylonitrile matrix (SAN matrix), which increases the adhesion of rubber with matrix and disperses rubber particles minutely, making monomer grafting butadiene rubber as a successful candidate for compatibilizer. The decrease of grafting reaction in the early reaction stage results in irregular disperse of rubber particles on styrene-acrylonitrile matrix. On the contrary, the excessive grafting reaction results in poor disperse of polymer solution during polymerization, leading to the decrease of fluidity and gloss of the final resin product. More precisely, when grafted chains make longer line, these chains push out styrene-acrylonitrile chains on matrix, inducing phase separation.
Therefore, a resin having excellent impact resistance and gloss can be prepared by regulating grafting reaction in the early stage and controlling rubber particles.
U.S. Pat. Nos. 2,694,692, 3,243,481 and 3,658,946 and EP No. 400479 describe that a polymerization method in which rubber is dissolved in aromatic vinyl compound and unsaturated nitrile compound and radical initiator is added to the reaction solution. The product by the method has excellent mechanical properties but poor gloss.
According to U.S. Pat. No. 4,640,959 and EP No. 103657, ABS resin having comparatively big particles 1.5 μm in average diameter has been prepared by solid phase polymerization, in which high viscosity butadiene rubber having 120 cp of solution viscosity was added to 5% styrene solution and conditions like monomer composition, reaction initiator, injection time and amount of molecular weight regulator and stirring speed were regulated. The product by the method shows excellent impact strength owing to the big particles but poor gloss.
U.S. Pat. No. 4,146,589 introduces the method of preparing a resin by polymerizing each rubber solution containing big particles and small particles in each reactor, respectively, which was then mixed. U.S. Pat. No. 4,254,236 describes another method for preparing a resin, in which rubber solution was polymerized in the first reactor and then other rubber solution having the same composition as the earlier one was polymerized in the second reactor to produce a resin whose rubber particles included bimodal.
U.S. Pat. Nos. 5,414,045 and 5,569,709 describe that ABS resin having excellent impact strength and gloss can be prepared by continuous bulk polymerization using a bulk polymerization reactor and a continuous stirred polymerization reactor, which facilitates the regulation of phase inversion and the up-regulation of graft copolymer content. However, to produce a resin with this method, special reaction equipment is required to increase the content of grafted side chain group, and the impact strength, gloss and tensile strength of the final product thereby are not so good.
U.S. Pat. Nos. 5,191,023, 4,587,294 and 4,639,494, and EP No. 277687 and Japanese Patent Publication No. 59-23214 and No. 59-179611 also introduce a method for producing a resin by using star-branched rubber. Japanese Patent Publication No. 5-194676, No. 5-247149, No. 6-166729 and No. 6-65330 and EP No. 160974 describe a method for preparing a bimodal resin, where small and big rubber particles are dispersed together, by using star-branched rubber and linear rubber. In the meantime, according to U.S. Pat. No. 5,569,709, grafting reaction was regulated by using a tubular reactor as a graft reactor, which resulted, though, in the excessive production of monomers in the early reaction stage, making the control of grafting difficult. Besides, the final product thereby showed poor balance between gloss and mechanical properties.